Not wishing to rain on anyone's parade here, and I have followed the discussion earlier re: the data sheets for the new cap, I would not have replaced the original capacitor with the newer smaller one.

Now I know that modern caps are superior to older ones, it was just that it was drummed into us as junior engineers that a much smaller cap physically should not be used to replace a physically larger one, as they will fail prematurely due to over heating.

I read the same advice again much later in Eugene Trundles excellent book "Servicing TV and Video Equipment", on the other hand Philips (especially) seemed to re-write this general rule of thumb by their prolific use of small disc high voltage caps in PSU snubbers and LOP stages, albeit in the very small nF range of values

So am I just being an old dinosaur that is stuck in the past using outdated teachings which may have been based on very valid (for the time) reasons ?

Anyone else offer any thoughts on this ? the capacitors physical size that is , not that I am a dinosaur lol , which is true beyond doubt btw

I think each case needs to be taken on its own. If the capacitor is going to pass a large AC current or high voltage spike then yes, go for similar size. On the other hand replacing a decoupling capacitor in a domestic radio from 70 years ago you could use a smaller type.

My rebuttal to that would be why has nobody ever mentioned this before on the countless other restorations here there and everywhere by countless restorers. For example we have all been happily replacing large waxies with those popular and tiny by comparison, yellow axial's. Likewise the huge sized electrolytic's of yesteryear with modern smaller 105 degrees. Almost all caps I've used in all my restores have been much smaller and nobody batted an eyelid.

Well the Waxies are an order of magnitude larger to their modern counter parts because of the differences between technologies, here we are using very similar technology in both capacitors, and I have mentioned "size matters" once before wrt to the G11 smoothing block.

As Frank (Nuvistor) points out it depends very much on the application, here, in that caps position we are talking about high pulse Voltages and currents, I know the more modern caps are using newer materials but the underlying technology is not that much different, unlike the case with waxies.

I could very well be wrong Chris, I looked at Gary's thread and the replacement cap is sized very similar to the original, the general rule of thumb was to change like for like, waxies as already pointed out are a different kettle of fish.

I was asking for others opinions on this too

Edit:Hi Gary your Cap was 5n1 and used as a replacement for the flyback tuning cap, I already said Philips used small disc caps in that position maybe the dissipation is not that high in the 11 microsecond flyback period, Chris's cap is 27nF and I would need to look closer at the position it is used, I don't think off hand 27nF is a typical flyback value. Thorn were known to be economical and I think they would have used a smaller cheaper cap in that position if they could.

As I say I am open to discussion.

Last edited by Red_to_Black on Fri Sep 09, 2016 6:57 pm, edited 1 time in total.

What that picture from the book doesn't tell you is what the dielectric is in the two caps. As I've said time and time again, polyester is for decoupling, polypropylene is for pulse work. I just dug out the datasheet that Chris has shown a page from - the cap can carry 1A (rms) at 20kHz, dV/dT spec is equally as impressive.

If any doubt it's back to manufactures spec sheets, if the specs state it will handle the load then that's ok. Unfortunately the specs very often don't cover what you need.and then it's what you know work.

Large AC currents, will tend to warm the capacitor and can be judged if ok. Large spikes are different, it will just breakdown without warning, some mixed dielectric capacitors common in late 60's early 70's had high working voltage, 1000Vdc but did not like line timebases with the spikes, or across the mains supply.

If you have found a type that works then carry on, that's experience, otherwise use caution.

nuvistor wrote:If any doubt it's back to manufactures spec sheets, if the specs state it will handle the load then that's ok. Unfortunately the specs very often don't cover what you need.and then it's what you know work.

Large AC currents, will tend to warm the capacitor and can be judged if ok. Large spikes are different, it will just breakdown without warning, some mixed dielectric capacitors common in late 60's early 70's had high working voltage, 1000Vdc but did not like line timebases with the spikes, or across the mains supply.

If you have found a type that works then carry on, that's experience, otherwise use caution.

Frank

That is partly what I was saying Frank.Cathy what those data sheets don't show is the duty cycle nor dissipation rates, as I am not a circuit designer and only a (former) mender I can only go by experience and my training, now I could well be wrong as I have said already.

I suppose the only way is to suck it and see as we are also not privvy to Thorns original design criteria or original reasoning for that size of cap, it could even been simply because that was the cheapest one obtainable at the time.

That is actually a very good point Malc It is true that the set will be unlikely to be switched on from morning until night everyday of the week.

I did download the full data sheet via the link kindly supplied by Catho, not sure I can fully take in all of the information contained within and also me being a bit thick couldn't translate the dT/dV spec. into anything like a simple yes/no it is/is not suitable.

Anyway more to the point I had a look at the circuit diagram, and the Cap C518 is indeed the line flyback cap, well the original value 0.028 μF seems a highish value compared to more modern sets where you would more usually see a max of maybe 15nF more usually about the 5 to 10 nF range also rated at 1400-1600V range as there is usually a 1.2kV 11μS duration flyback pulse, but I digress.

Edit: The way I was actually taught with general rules of thumb (and bodgery ) in the line flyback dept. was the replacement cap should be physically similar in size (this discussion), have at least the same or better voltage rating eg a 1.6kV cap can replace a 1.2kV but not vice versa, it was ok to use the next value up but not down.

That last bit was explained as follows, if you reduce the capacitance value the flyback pulse gets narrower but higher in peak voltage, if you increase the value the pulse gets wider but lower peak voltage, but this is also a trade off, if you increase the width/lower peak you also keep the output transistor switched on in it's transition period for longer (slowed the pulse down) and so it dissipates more heat.

This is sort of between a rock and a hard place, ie. you reduce the value too far and you risk high EHT and problems with punch through on the lop transistor, transformer and other related components, increase it too far and risk the lop transistor running too hot and failing, both conditions are not good for reliable long term repairs.

Ps. similar general rules of thumb apply to the snubber caps in Switched Mode PSUs.see, I am a mine of useless information lol

Red to black wrote:Ps. similar general rules of thumb apply to the snubber caps in Switched Mode PSUs.see, I am a mine of useless information lol

On the contrary, it was very informative and you did touch upon dV/dT yourself in not as many words.

The issue of pulse width was once the root of a fault in my old Telefunken 711, which was tripping due to excessive EHT. Investigation led me to a timing component around the TBA950, which was causing the line drive waveform not to be the correct 28:36 mark/space ratio, but something like 12:52. As R2B pointed out, this plays havoc with the EHT voltage in a flyback system. Replacing the offending component restored normal operation.

I've had the set on for 15mins and the new flyback tuning cap is cold to the touch. I will leave it even longer but certainly I believe its more than up to the job.

Back to the matter in hand, after replacing the blown resistor R528, replacing C511 and reinstating the broken beam limiter earth to LTB. No change still on hoz lock. The tuner has all but fallen apart and may not be helping three of the selectors are locked in and one seems to tune but I'm not happy with it. I might see if I have a spare tuner in stock, then again I might just put this to one side.

I've had the set on for 15mins and the new flyback tuning cap is cold to the touch. I will leave it even longer but certainly I believe its more than up to the job.

When the set's complete, run it for an evening. I'm sure it'll be just fine and dandy.

CrustyTV wrote:Back to the matter in hand, after replacing the blown resistor R528, replacing C511 and reinstating the broken beam limiter earth to LTB. No change still on hoz lock. The tuner has all but fallen apart and may not be helping three of the selectors are locked in and one seems to tune but I'm not happy with it. I might see if I have a spare tuner in stock, then again I might just put this to one side.

I think it may be instructive mechanically to persevere with it; but if you have a spare, use that and use this one as a learning process for mechanical overhaul.

But first, the lack of line hold... can you vary the speed of the line timebase?